Recovery from Friend virus (FV)-induced leukemia in mice is influenced by the Rfv-1, Rfv-2 and Rfv-3 genes. The H-2D-linked Rfv-1 gene appears to affect the FV-specific T lymphocyte response kinetics. Current data using monoclonal anti-L3T4 and anti-Ly2 antibodies indicate that the main T cell response detected in vitro from both high and low recovery phenotype mice (H-2Db/b and H-2Dd/b) involves helper T lymphocytes, which recognize antigen together with I-A gene products. Influence of the H-2D region on this response has not been noted previously in other systems. In other experiments, mice with the bm14 mutation in the H-2D region were found to have a decreased incidence of recovery from FV leukemia. A new fluorescent immunoassay using monoclonal antibodies to detect foci of viral infection in live cell monolayers has been developed. This assay has facilitated quantitation of retroviruses, rabies virus and herpes simplex virus, as well as permitted rapid biological cloning of viruses in complex mixtures.